Polycrystalline metal oxide varistors, commonly known as MOV's, are well known in the art. MOV's include metal electrodes separated by sintered ceramics comprising a variety of metal oxides, zinc oxide being the predominant ceramic with lesser quantities of other oxides added in, including but not limited to oxides of bismuth, manganese, cobalt, antimony and/or tin. The metal electrodes may be made of any conductive material and are typically disposed on opposed major surfaces of the ceramic substrate.
MOVs commonly have the geometry of a circular disc shape with a thickness much smaller than the radius of the disc. A generic embodiment of a prior art MOV is shown in FIG. 1, wherein a ceramic substrate 11 in the shape of a disc separates a circular shaped first electrode 14 from a circular shaped second electrode 18. Such disc-type MOV's are typically coated with a non-conductive material to prevent arcing between the electrodes about the cylindrical sides of the disc.
MOV's are provided in electrical parallel with a parent electrical circuit. Current travels, if at all, from one electrode to the other through the ceramic substrate, which acts as a variable resistor (varistor). The principal advantage of MOV's is that the electrical conductivity of the ceramic substrate changes non-linearly with respect to the voltage applied. The voltage at which an MOV's electrical conductivity dramatically changes is referred to as the clamping or breakdown voltage. When the applied voltage is below the threshold or clamping voltage of the MOV, the device acts as an open circuit and virtually does not conduct. When the device is electrically connected in parallel with a parent circuit, and an over-voltage condition occurs (as often happens during a surge), the voltage may rise well over the nominal operating voltage of equipment located in the parent circuit. When this surge exceeds the clamping or breakdown voltage, the MOV's ceramic substrate will breakdown electrically, thus creating a virtual short circuit in parallel with the load; conducting the surge away from the parent circuit and associated protected equipment. MOVs behave electrically much like two Zener diodes facing each other in series. Like such an arrangement, MOVs are bi-directional.
The electrical properties of MOV's may be described by the following equation: ##EQU1##
wherein:
I is the current through the MOV, PA1 V is the voltage across the electrodes, PA1 C is a constant dictated by the substrate material and its geometric configuration, and PA1 .alpha. is a constant for a particular range of current across the electrodes.
Regarding the constant C in the above equation, the clamping voltage of a particular MOV is a function of the thickness of the particular substrate material interposed between the electrodes. Thicker substrates exhibit higher clamping and breakdown voltages. However, the amount of surge current that a particular MOV can effectively dissipate also is a function of the surface area of the electrode/substrate juncture. If the surge current is too great for this surface area and for the mass of the varistor substrate, the device will be destroyed due to its inability to dissipate the surge energy and the high impedance that may be posed by the insufficient surface area of the electrode/substrate juncture. This destruction often results in a catastrophic failure of the varistor device, and depending on the mode of failure may also result in a condition known as thermal runaway. While prior art MOV's encompass a wide variety of clamping voltages, many are limited in their ability to carry significant current capacities. In order to carry higher currents, the radius of disc-shaped MOVs must be increased. This is undesirable because of the extra space such an MOV would occupy in a circuit board for example. Thus, what is needed in the art is a metal oxide varistor of more compact shape that can dissipate higher currents without undergoing thermal runaway and/or catastrophic failure.